Magnetic bubble domain devices are well known in the art. The most familiar mode of operating a magnetic bubble device is termed the "field-access" mode. In this mode, a pattern of magnetically soft elements (such as Permalloy) is formed in a plane adjacent a layer of material in which the bubbles are moved. A magnetic field is generated in the plane of the layer and the field caused to reorient to incrementally-offset radial positions cyclically in the plane. Each element is so shaped that various portions thereof respond to the in-plane field to generate pole-patterns which change as the field precesses. The configuration of adjacent elements sets up a sequence of travelling potential wells in the layer which causes bubble movement.
Since data is represented by such a small entity--the magnetic bubble--in a device of this type, the bubble must be expanded laterally with respect to the axis of movement of bubbles in its propagation path. An elongated bubble is necessary in order for the bubble to make a sufficient change in a magneto-resistive detector incorporated in the propagation path to achieve an adequate output signal level from the detector. The preferred technique used to increase the bubble size as the bubble is propagated involves the use of increasing numbers of Permalloy elements in a progression of stages corresponding to the bubble propagation path which leads up to the magneto-resistive detector. Such an arrangement is commonly referred to as an "expander detector". Various types of chevron stretcher detectors used in magnetic bubble domain systems are known. Although these chevron stretcher detectors are satisfactory for most commercial applications, the failure of bubble propagation has been observed on a bubble propagation path at the rapid expander transition at the point where an asymmetrical chevron meets the chevron expander. The same failure mode has also been observed on the gradual transition expander design occurring typically at the points where the strip expands to the next chevron stack. In such a case the failure is more severe when the stretch differential between the chevron stacks is increased. The failure is also more severe as the frequency is increased. It has been suggested that such failure is associated with the redeposition that can result from the ion milling of the Permalloy propagation pattern. Redeposition forms very thin walls of permalloy possible mixed with oxide on the periphery of the propagation elements. These walls are apparently magnetic and can reduce the effective magnetic pole strength and the drive on the bubble. In the case of the expander this reduced drive is more apparent in the regions of the expander where chevrons are not located next to other chevrons. Although removal or elimination of the redeposition essentially eliminates the failure over nominal operating conditions (0.degree. C. to 100.degree. C. at 150 kHz), it is possible that propagation reliability in the stretcher may still be a problem at higher frequencies or at lower temperatures, since free ended chevrons pose a different environment to the stretching bubble and possibly a lower drive.